ARM64: configs: Add Platform MHU in defconfig

Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NKevin Hilman <khilman@baylibre.com>

arch/arm64/configs/defconfig

@@ -396,6 +396,7 @@ CONFIG_MSM_MMCC_8996=y
 CONFIG_HWSPINLOCK_QCOM=y
 CONFIG_MAILBOX=y
 CONFIG_ARM_MHU=y
+CONFIG_PLATFORM_MHU=y
 CONFIG_HI6220_MBOX=y
 CONFIG_ARM_SMMU=y
 CONFIG_QCOM_SMEM=y